What is breast density?
Breast density is a unique attribute of the breast that can only be determined with an X-ray (mammogram). It is the degree to which the breast tissue prevents X-rays from passing through and therefore dense breast tissue appears white on a mammogram.
Everyone’s breast is a mixture of glandular, fibrous, and fatty tissue. Fatty tissue lets the X-rays pass through and appears dark grey, while glandular and fibrous tissues are denser tissues, so the X-rays don’t easily pass through and these tissues appear white on a mammogram.
Every woman has her own unique mix of fibroglandular and fatty tissue that makes up her breast density. Your breast density is included in your mammogram report. Beginning January 15, 2017, you will also receive your breast density reading in your lay summary that is mailed directly to you.
Why is breast density important?
Breast density is important because we detect cancers most frequently when they stand out as white spots on a dark background. If your background also appears mostly white (as very dense breast tissue would), it is more difficult to detect tumors. The ability to detect tumors is significantly less in women with dense breasts. So, there is a greater chance that a screening mammogram could miss a cancer, if present, when the breasts are dense.
There is also evidence that women with dense breast tissue have a slightly higher risk of breast cancer than women with fatty breasts. The risk of cancer in a woman with heterogeneously dense breasts is about 1.5-2x the risk for an average woman without dense breasts and the risk for a woman with extremely dense breasts is about 2-2.3 x the risk of an average woman. This increased risk is relatively small though when compared with other risk factors like age, family history, or a prior biopsy that showed atypical cells.
How many women have dense breast tissue?
It’s important to know that having dense breasts is not in any way abnormal! Approximately 40-50 percent of the population has dense breasts. Dense breasts are even more common in younger women and somewhat less common in older women.
What can be done to improve cancer detection in women with dense breast tissue?
Digital mammography improves cancer detection in women with dense breasts compared with the older film screen mammography. 3D mammography improves detection further – and has the added advantages of reducing false positive results. So, a 3D mammogram is a good way to improve early diagnosis if you have heterogeneously dense breasts. Unfortunately, none of these methods detects ALL breast cancers and there are still some cancers in dense breasts that are not detected with mammography, especially in those with extremely dense breasts.
Some women with dense breasts may wish to consider supplementary screening in order to improve the detection rate for cancer. The two most common supplementary screening options are ultrasound and MRI. MRI is usually recommended for women at elevated risk for breast cancer (>20 percent lifetime risk) regardless of their breast density. Ultrasound can be used to supplement mammography screening in women at less than 20 percent lifetime risk, if they wish to pursue additional testing. Ultrasound detects more small invasive cancers than mammography, but it has a higher false positive rate. Many biopsies and follow-ups are required for every cancer detected, although the false positive rate will decrease when the exam is repeated on a regular basis.
What can I do?
If you want to be proactive about your health care, pay attention to your breast density. This is a good opportunity for women of all densities to discuss their lifetime risk of breast cancer with their provider. A risk assessment can determine whether you are low, moderate, or high risk. If high risk, you should have MRI screening to supplement mammography. Depending on your family and personal history, you might also consider genetic testing and/or taking medication to decrease your risk.
For women with dense breasts and no other risk factors, no medical organization is currently advocating supplementary screening for women with dense breasts. But if your goal is to do everything you can to make sure that if you develop cancer, it is caught early, and you have dense breasts, you might choose to add a test to mammography to increase the chance for detection. You should be aware that there may be false positives. Decisions should take into account the patient’s values and philosophy. Not all supplementary screenings are covered by insurance but this may change over time. There are no straightforward answers for women with dense breasts. There are many helpful resources and websites. Check out our breast density information page to learn more about breast density.
Ultimately, there is nothing a woman can do about her breast density or genetics. Living a healthy lifestyle, maintaining a healthy weight, getting regular exercise, and minimizing alcohol consumption are all things we can do to prevent breast cancer.
Sally Herschorn, MD, radiologist, is Medical Director of Breast Imaging at the University of Vermont Medical Center.